What makes people gain extra body fat?

Having extra body fat is a relatively modern problem. For the previous 10,000 years or so, the major concern for humans had been to find enough food, not having to deal with too much of it. As a result, your body's physiology evolved to conserve body fat, because fat is a very efficient way for your body to store energy and to easily convert dietary fat calories into body fat. In a world of scarcity, those who gained weight easily had an evolutionary advantage; it was 'survival of the fattest.' One hundred fat calories can be stored as body fat by expending only 2½ calories, whereas your body must spend 23 calories, almost ten times as much, to convert 100 calories of dietary protein or carbohydrate into body fat. Only about one percent of dietary protein and carbohydrates ends up as body fat, because your body would rather use them up right away, rather than wasting energy to store them. Therefore, by keeping fat consumption low, not only do you tend to consume fewer calories, but also those calories are less likely to be converted into body fat. Most people in the U.S. today also eat a lot of simple carbohydrates-- such as white-flour products, white rice, sugar, and alcohol-- which are absorbed quickly, causing your blood sugar to rapidly increase because the fiber and bran have been removed. In response, your body secretes insulin to lower your blood sugar levels to normal, which accelerates the conversion of calories into fat, raises your cholesterol level, and has other harmful effects. Consumption of sugar, white flour, and processed foods has increased significantly in the past two decades, along with the rate of obesity. Social forces have also contributed, such as the rise in popularity of fast food and video technology, the cutback or elimination of physical education in schools, and adults becoming 'couch potatoes.